The government has given the green light, as they put it, for the construction of the two 65,000-tonne aircraft carriers, the Queen Elizabeth and the Prince of Wales, to go ahead. Once BAE and Vosper Thorneycroft can put together the consortium of five partners involved in the build, they'll get the contract for about £4 billion.

In truth, it would probably be better to halve that sum and multiply by 10 - because this project, if it is ever completed as envisaged, is likely to cost £20bn or above. It's a fairly staggering sum; particularly when the armed forces are going to be told in six weeks' time to slash their budgets to reduce the £2bn overspend on equipment forecast for this and the next two years.

So what is going on here? It is less Alice's Adventures in Wonderland, and more in keeping with the rigorously surreal logic of Through the Looking Glass, where, you may recall, Humpty Dumpty had words mean whatever he paid them to. And statistics and numbers, it seems when it comes to this government's defence budget forecasting. Defence cost inflation is much higher than in much of the civilian world; and the government's forecast of spending £3.9bn to build just the ships, without the systems and the aircraft, seems sufficiently optimistic as to make Pollyanna appear the hardest-bitten realist.

The government's release about the "green light" for the carriers doesn't mention price, but it does mention another figure. It says the project will create 10,000 jobs in Britain's hard-pressed ship construction industry. A fair slice of those jobs will be at Babcock's on the Clyde, where the dock is now being revamped, and at the main fitting-out facility at Rosyth on the Firth of Forth - not unadjacent to the parliamentary constituencies of Messrs Brown (PM) and Browne (defence secretary).

The carriers are the biggest construction project in the navy's history. In cost it will surpass the build of the current Trident ballistic missile system and its four Vanguard submarines.

As a burden on the defence budget, and the nation's purse, it will rival the two of the biggest defence procurements since the end of the second world war; the Panavia Tornado air defence and strike aircraft, and the still incomplete Typhoon Eurofighter procurement, costing £22bn and climbing.

The carriers themselves are huge, and there must be some question about why anything so big is needed for Britain's maritime service today. You could fit all the personnel of the current Royal Navy and Marines and much of Nelson's besides into them easily. There has been some serious question about whether the navy, now at 38,900, can really fully man and maintain such large and complex ships and systems, and the submarines, the helicopters, the surface fleet, and the Royal Marines besides.

The biggest problem, perhaps, lies with what it says on the extremely large grey tin of this project. The purpose of an aircraft carrier, is for it to launch a variety of "aerial platforms" - aeroplanes to you and me - from its decks. The maxim is that you build the ship round the type of aircraft you want, rather than the other way round.

The aircraft intended for the ships, or for which the ships are intended rather, is the Lockheed Martin F-35 Joint Strike Fighter (JSF). It is the single most expensive aircraft project in history, with a budget climbing into a clear blue sky at around $250bn - and showing no sign soon that it will throttle back into level flight. Furthermore, like most such big procurement items, it is well behind schedule. If the Queen Elizabeth and the Prince of Wales are commissioned to schedule in 2014 and 2016, the F-35 won't be anywhere near ready, and the navy will make do with the RAF's GR9 Harriers up until 2020 at least.

The problem for any defence purchases from the United States is that there they do cost overruns as if they are going out of fashion by comparison with European competitors. A prime example is the floating gun platform, a sort of amphibious tank, being developed by General Dynamics called the expeditionary fighting vehicle. It's currently driving the budget scrutinisers in Congress wild. It is 11 years late, and won't be ready for some time. Initially, 1,025 vehicles were ordered for the US marine corps in 2000, for $7bn. The Pentagon has now agreed to buy only 593 vehicles for a revised price of $15.8bn - and trials to date have shown up 168 major faults in the prototypes.

By the time the carriers are up and running, with the aircraft for which they are intended in about 2020, the whole concept of using such platforms to launch manned strike aircraft may be facing obsolescence. New unmanned aircraft and standoff bombs and missiles, which might be launched hundreds of miles from their targets, could be dominating the aerial battle space.

One thing for which the carriers will be extremely useful is as a sea platform from which to shift large amounts of aid and reconstruction material in major disasters like the Burma cyclone and the Christmas Tsunami that devastated Sri Lanka, Indonesia and Thailand. But this will require a big shift in doctrine and training for the services, and the politicians. And it is a very expensive way to provide this kind of support ship.

The carrier procurement not only suggests that the government doesn't understand the difference between the anatomical fitting above its thigh and its mid-arm joint - but that its tight-fisted right hand doesn't know its strangely profligate left.